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Paper

Can Minimal Cerebellar Tonsillar Ectopia Can Be Symptomatic?

Dan S. Heffez MD, FRCS, Jack D. Clemis MD, Dean A. Elias MD, Michael A. Mikhael MD, PhD

Running title: Minimal Tonsillar Ectopia

Abstract

Objective: Tonsilar ectopia of 3-5mm or more is believed to respresent a symptomatic Chiari type 1 malformation. We questioned if tonsillar ectopia of less than 3mm below the foramen magnum could be symptomatic.

Method: Retrospective case series review of 28 patients with signs and symptoms of cervicomedullary compression in whom the mid-sagittal MRI image demonstrated tonsillar ectopia of less than 3.0 mm below the rim of the foramen magnum, but in whom radiographic evidence for cervicomedullary compression was evident on axial MRI images oriented through the plane of the foramen magnum. Patients underwent sub occipital decompression, with or without C1 laminectomy and duraplasty.

Results: Seventy-eight percent of patients were female. The mean age was 41.6 years. The mean duration of illness was 44 months. The mean tonsillar ectopia as measured on the mid-sagittal MRI image was 0.57+/-1.95mm, well below the accepted threshold for diagnosis. The most common symptoms are headache (93%), gait instability (82%), chronic nausea (82%), cognitive impairment (82%), and neck pain (68%). The most common neurological findings were hyper-reflexia (61%), dysmetria (46%) and a positive Romberg sign (43%). At the time of surgery, tonsillar ectopia was noted to be more extensive in the parasagittal than the mid-sagittal plane, a finding which would be overlooked by current MRI criteria for the diagnosis of Chiari type 1 malformation. Following surgery, patients reported improvement in a number of symptoms including headache (82%), gait instability (82%), nausea (67%), cognitive difficulty (64%), dizziness (57%), body pain (50%) and numbeness (46%), (minimum follow up = 6 months; mean follow-up=12.8 months).

Conclusion: A minor degree of tonsillar ectopia may produce signs and symptoms of cervicomedullary compression which can improve following surgical decompression. The current MRI criteria for the diagnosis of symptomatic Chiari type1 malformation should be re-examined as they may be too restrictive.

Key Words: cerebellar tonsil, ectopia, Chiari type1 malformation, magnetic resonance imaging, diagnosis

Introduction

The Chiari type 1 malformation is characterized by the displacement of the cerebellar tonsils through the foramen magnum and into the cervical spinal canal, likely the result of a volumetrically small posterior fossa (6,12,13,14,18,21). Acquired tonsillar ectopia has also been reported in relation to lumbar cerebrospinal fluid (csf) drainage (4,8,9,15,17,22). Patients may be asymptomatic, have extensive symptoms with only subtle neurological signs or be incapacitated secondary to severe headache, vertigo, ataxia or the consequences of secondary hydromyelia(3,5,6,7,19). Currently the diagnosis of the Chiari malformation is made using magnetic resonance imaging (MRI)(2,3,6,10,16,20,23). With the advent of MRI scanning, it has become apparent that some degree of tonsillar ectopia is a common finding and need not be symptomatic(3,4,6). Accordingly, attempts have been made to set MRI criteria for the diagnosis of the Chiari type 1 malformation that are not too sensitive. These criteria currently are(2,3,4,6,10,16,20):
1) herniation of at least 1 cerebellar tonsil 5 mm or more below the plane of the foramen magnum as defined on the sagittal T-1 weighted MRI image.
2) herniation of both cerebellar tonsils 3-5 mm below the foramen magnum if accompanied by other definite radiographic features associated with the Chiari malformation (e.g. syringomyelia).
3) ectopia of 2 mm or less is unlikely to be of clinical significance in the absence of syringomyelia (3).
4) "In cases of marginal ectopia(3-6 mm) evidence of tonsillar distortion must be present before the diagnosis of Chiari 1 malformation is made." (2)

The rigid application of these criteria in the face of symptoms and neurological signs that could be localized to the cervicomedullary junction can lead to the under diagnosis of an eminently treatable condition. We have evaluated 28 patients with symptoms consistent with compression of the neuraxis at the foramen magnum and in whom the MRI scan was either reported as normal or showed a degree of tonsillar ectopia insufficient to meet the diagnostic criteria for the Chiari type1 malformation. The clinical presentation, radiological findings, management and outcome in this series of patients is the subject of this report.

Methods

Patient Selection and Follow-up
Most patients were referred for evaluation of headache associated with gait disturbance or otological symptoms. Some symptomatic patients were referred because of a suspected Chiari type 1 malformation on the basis of MRI imaging. A neurological evaluation was performed by the neurosurgeon (DSH) in accordance with standardized protocol designed to identify signs of cervical myelopathy and lower cranial neuropathy. Initial follow-up was conducted by direct interview and examination while subsequent follow-up was often conducted through mail by questionnaire; many patients lived a great distance from our institution. The questionnaire asked patients to designate individual symptoms as resolved, improved, unchanged or worsened.

As otological symptoms are common in patients with the Chiari type1 malformation(7,19,24), formal neuro-otological evaluation was undertaken whenever possible. Evaluation included computerized dynamic posturography, electronystagmography, and audiometry. One patient presented initially to the neuro-otologist because of complaints of disequilibrium and vertigo that were judged to be of a central origin.

Radiological evaluation
Every patient underwent MRI imaging in accordance with a protocol. MRI scans were performed on a Siemens Magnetom or a General Electric Signa system, 1.5 Tesla field strength, 256x256 matrix. Using the Seimens scanner, standard axial, sagittal and coronal T1 weighted images, (6mm thick with a 5% gap, TR= 550-600msec, TE=14msec, 60% flip angle) were obtained. T2 weighted axial images were also obtained, (6mm thickness, 5% gap, TR=5700msec, double echo TE=14 and 90msec). Additional T2 thin axial sections were taken through the plane of the foramen magnum, (3mm thickness, 0.1mm gap, TR=5000msec,TE=90msec). Similar scanning parameters were used with the General Electric scanner. The oblique T2 weighted images through the foramen magnum were obtained using a 2D spin echo fast spin echo technique, (TE=85, TR=3600, 3mm slice thickness, 0.5mm gap, 256x256 matrix). It was felt that as the cerebellar tonsil is a parasagittal and not a mid sagittal structure, a mid sagittal image could conceivably fail to fully demonstrate the degree of tonsillar ectopia, (fig. 1 and fig. 2). Furthermore, it was also felt that the extent of cerebellar ectopia per se may be of lesser significance than the extent of crowding at the level of the foramen magnum, a situation which would best be appreciated on the axial images. For the purpose of determining the position of the cerebellar tonsils the lower lip of the foramen magnum was defined as extending from the lowest cortical bone of the clivus anteriorly (basion) to the lowest cortical bone at the opistion posteriorly on the mid-sagittal MRI image(3,6). The position of the most caudal point of the tonsil(s) relative to the inferior lip of the foramen magnum was measured from the mid-sagittal MRI slice as is the convention(6). If the the cerebellar tissue was positioned rostral to the plane of the foramen magnum, the position was expressed as a negative value in mm. If the cerebellar tissue was located within the foramen magnum the position was given a value of 0. Caudal displacement ws expressed in mm with a positive value. Only patients with caudal displacement of the cerebellar tonsils of less than 3.0 mm were included in this series. It was felt that a greater degree of displacement would be recognized as abnormal by currently accepted radiological criteria for diagnosis. The axial MRI images were examined for evidence of crowding of the neural and vascular structures. The absence of a csf signal dorsal to the ectopic cerebellar tonsils, or distortion of the brain stem, cervical spinal cord or cerebellar tonsils was taken as evidence of physical compression of the contents of the foramen magnum.

MRI of the cervical spine was routinely performed to exclude other pathology that might be responsible for the patients symptoms. In 14 patients radiographic evidence of mild to moderate congenital cervical stenosis was noted.

Surgical treatment
Surgical treatment was prescribed on the basis of a reproducible clinical syndrome of symptoms and objective signs. Supportive neuro-otological evidence for a central etiology of vertigo or dizziness was available in 6 patients. Radiographic evidence of tonsillar ectopia in the absence of a convincing clinical picture was not sufficient to prescribe surgical therapy. Surgical treatment involved sub-occipital craniectomy to enlarge the foramen magnum. The rim of the foramen magnum was resected laterally as far as the atlanto-occipital joint bilaterally. The arch of the atlas was removed if the cerebellar tonsils extended below the rim of the foramen magnum but otherwise was left intact. Duraplasty was performed using either human cadaver dura (Transplantation Research Foundation, Bellaire, Texas), (5 cases) or bovine pericardium (Duraguard, Biovascular Inc., Saint Paul, Minnesota), (19 cases). In four patients craniectomy without duraplasty was performed. In these patients B mode ultrasound demonstrated a generous dorsal subarachnoid space following craniectomy alone. The arachnoid membrane was widely opened in six patients but left grossly intact in the remaining patients undergoing direct decompression of the foramen magnum in order to avoid pseudomeningocele formation. One patient who had been previously diagnosed with pseudotumor cerebri on the basis of partial sagittal sinus thrombosis was obliged to undergo revision of lumboperitoneal shunt to ventriculoperitoneal shunt in addition to suboccipital decompression.

Results

Clinical presentation
Patients ranged in age from 15-79 years with a mean of 41.6+/-12 years. There were 22 female and 6 male patients. All patients were initially evaluated because of specific clinical complaints; no one was referred solely on the basis of MRI findings. Four patients had previously undergone uneventful treatment for other neurological disorders, [aneurysmal subarachnoid hemorrhage or surgical clipping of unruptured aneurysm(n=3), lumboperitoneal shunting for pseudotumor cerebri(n=1)]. Fourteen patients presented with new onset of symptoms after apparently trivial head trauma. The duration of symptoms prior to evaluation ranged from 6 weeks to 15 years(mean 44.38 months).

The presenting clinical symptoms are listed in table 1. The constellation of symptoms was typical of that seen with Chiari type 1 malformation (5,12). The most common complaints were headache, neck and arm pain, cognitive impairment, instability of gait, chronic nausea and dizziness. Of note, 64% (18/28) of patients reported increased symptoms with neck extension, a maneuver which would be expected to increase cervicomedullary compression. In general, objective neurological findings were subtle in this group of patients (table 2). The most common physical finding was hyper-reflexia (17/28 patients, 61%). Intolerance of neck extension was noted in 22 of 28 patients, (78%). This maneuver resulted in an exacerbation of the patients existing symptoms and an accentuation or provocation of hyper-reflexia, dysmetria, disdiadokokinesia and nystagmus.

Neuro-otological evaluation
Six patients underwent neuro-otological testing. The results of preoperative neuro-otological testing are summarized in table 3. All patients with complaints of vertigo were found to manifest this symptom with head on body extension exclusively and independently of all other positions commonly associated with positional vertigo. This was the only consistent preoperative finding. No patient presented with the subjective complaint of hearing loss. However, 1 patient developed a flat left 45 decibel sensorineural hearing loss following sub-occipital decompression that recovered completely following repair of a pseudomeningocele.

Radiological studies
The average tonsillar ectopia in mm below the inferior rim of the foramen magnum as judged from mid-sagittal T-1 weighted MRI views was 0.57+/-1.95mm. In 14 cases, the tonsils did not descend below the level of the foramen magnum as demonstrated on the mid sagittal MRI slice. In these 14 cases the axial thin sections through the plane of the foramen magnum contributed additional information by demonstrating clear tonsillar herniation lateral rather than dorsal to the cervicomedullary junction. Evidence of compression of the brain stem between dorsolaterally displaced cerebellar tonsils and ventrally positioned vertebral arteries was noted in every case. In 6 of these 14 cases, the mid-sagittal MRI image showed the tonsils to be above the level of the foramen magnum. In each of these patients caudal ectopia of the tonsils was appreciated on parasagittal images and on the appropriate axial view.

MRI imaging performed after decompression confirmed dorsolateral decompression of the neural structures and restoration of the dorsal CSF space.

Surgery
Surgical decompression afforded improvement or resolution of many symptoms, (table 4). Improvement of abnormal neurological signs was noted at the 1 month follow-up visit, (table 5). Signs of cerebellar dysfunction or disordered extra-ocular motility resolved completely, (table 5). Hyper-reflexia persisted as a finding in most patients. The minimum documented post-operative follow-up was 6 months with a mean follow-up of 12.8 months.

In 6 patients the arachnoid had been opened and the intradural anatomy inspected for evidence of compression such as flattening of the accessory spinal nerve, grooving of the cervico-medullary junction and/or cerebellar tonsils by the vertebral or posterior inferior cerebellar arteries, or flattening of the cerebellar tonsils. In subsequent cases the arachnoid was not widely opened in an attempt to minimize the likelihood of pseudomeningocele formation. More pronounced tonsillar ectopia was noted in the parasagittal than in the mid-sagittal plane in each case either by direct inspection when the arachnoid had been opened or by the use of intra-operative B-mode ultrasound. There were no neurological complications or post-operative infections. One patient developed a large pseudomeningocele which required surgical repair.

Illustrative cases

Patient #3: a 40 year old female, suffered a subarachnoid hemorrhage October 1993 due to a ruptured right posterior communicating artery aneurysm . The postoperative course was uncomplicated. Despite a long history of migraine headaches she described a new constant pounding, nauseating headache and a sensation of dizziness and rotation when lying on her left side. The headache was left retro-orbital in location and extended to the vertex. Headache was classified by a neurologist as migraine and muscle tension type and she was treated accordingly. In June 1994 she began to complain of increasing vertigo and dizziness with neck extension, frequently occurring in conjunction with exacerbations of neck pain. Discomfort extended to the shoulders and over the occiput to the vertex and the frontal region. She reported spontaneous gait instability. Physical examination was remarkable for immediate onset of severe vertigo with neck extension. MRI showed 2.9 mm of tonsillar descent which had gone unreported (fig. 3). In 1993 an otolaryngologist had diagnosed non-specific vertigo. Repeat otolaryngological evaluation could not identify any vestibular basis the vertigo. The patient underwent a sub occipital craniectomy, c-1 laminectomy and duraplasty. Vertigo improved immediately and resolved within 1 month of surgery. The patient continued to have some headache associated with nausea but of much less severe intensity. At the 8 month follow-up while some headache persisted, vertigo had not recurred.

Patient#6: a 15 year old boy who experienced several episodes of loss of consciousness due to tachycardia. In February 1996 he felt hot and dizzy and fell striking the back of his head. He noted the onset of occipital headache extending to the neck, tenderness over the scalp in the right occipital region, true vertigo associated with nausea, persistent bilateral tinnitus, and paroxysmal blurred vision associated with exacerbations of headache. Transient paresthesiae of the entire body were noted at the time of the fall. Physical examination revealed only right end gaze nystagmus. MRI scan of the brain was unremarkable with the exception that the cerebellar tonsils were within the posterior foramen magnum (0 mm of tonsillar ectopia), (fig. 4). The case was reviewed by 3 experienced neurosurgeons and 2 neurologists who diagnosed post-traumatic migraine headache unrelated to the tonsillar ectopia. The patient was treated for migraine during 1 month. He reported occasional episodes of severe disorientation, increasingly severe headache associated with vomiting, visual spots, occasional blurred vision, shoulder and interscapular pain, and tinnitus. Physical examination revealed mild blurring of the optic disc margins, asymmetric disdiadokokinesia, and upper extremity dysmetria. Symptoms were not aggravated by neck extension. Formal speech pathology evaluation revealed pharyngeal dysphagia and mild lingual weakness. Suboccipital craniectomy, C1 laminectomy and duraplasty was performed. At surgery there was evidence of long standing compression as evidenced by the flattened pale appearance of the accessory spinal nerve and the vascular grooving of the caudal brain stem by the posterior inferior cerebellar artery. Relief of symptoms was prompt and complete. The patient continued to do well 36 months following surgery.

Discussion

The radiographic criteria by which the diagnosis of the Chiari type1 malformation should be made have long been a point of debate(2,3,4,6,10,16,20). With the availability of MRI and the attendant ease of imaging the posterior fossa, diagnostic criteria had to be established which would have sufficient sensitivity and specificity to differentiate between symptomatic and likely asymptomatic cases of tonsillar ectopia. The threshold for diagnosis has relied solely on the degree of tonsillar ectopia measured in mm below the plane of the foramen magnum as judged on the mid-sagittal T-1 weighted MR image. As much as 5 mm of ectopia has been noted in patients having "symptoms unrelated to the Chiari-1 malformation"(3). The mean "normal" position of the cerebellar tonsil has been stated to be 1+/- 1.9 mm above the foramen magnum(3). Findings such as these have led to the accepted view that 2 mm or less of tonsillar ectopia is unlikely to be of clinical significance in the absence of syringomyelia(3). More recently the variation in the degree of acceptable tonsillar ectopia in relation to a patient's age has been examined. During the second and third decades of life up to 5 mm of tonsillar descent has been considered normal. Between the ages of 30 and 79 years up to 4mm of tonsillar ectopia has be considered within normal limits(11). In our experience, limiting the diagnosis of the Chiari type 1 malformation to cerebellar tonsil ectopia of 3-5 mm or greater below the rim of the foramen magnum(2,3,4.6,10,16,20) may be too restrictive. Patients with lesser degrees of tonsillar ectopia who complain of symptoms consistent with compression of the structures coursing through the foramen magnum could potentially be denied the benefits of decompressive surgery.

We have identified 28 patients who have presented with one or more symptoms associated with the Chiari type 1 malformation (5,12) but in whom the degree of tonsillar ectopia as judged from the MRI scan did not meet the conventional threshold for diagnosis. In 14 patients the onset of symptoms was attributable to a minor closed head injury. In these patients the associated transient increase in intracranial pressure might be expected to have promoted further tonsillar impaction at the level of the foramen magnum. The average degree of tonsillar descent below the level of the foramen magnum in our group of patients was 0.57(+/-1.95) mm as judged on the mid-sagittal T1 weighted MRI image, well below the accepted threshold for diagnosis. This implies that in some patients the cerebellar tonsils were not herniated at all as judged from the mid-sagittal MRI image. In each case tonsillar ectopia and brain stem compression were best demonstrated on the axial image through the plane of the foramen magnum. Despite the minimal degree of tonsillar ectopia, our patients reported improvement in symptoms in response to surgical enlargement of the foramen magnum. An improvement in objective neurological signs was also noted. While there was no unoperated control group of patients for comparison, our patients might be considered to have served as their own controls. Prior to our intervention they had been symptomatic for an average of 44 months.

Milhorat et al, in a report of 364 patients with symptomatic Chiari type 1 malformation, identified 32 patients with tonsillar ectopia less than 5 mm. Seventeen of these patients also had syringomyelia. They concluded, "the extent of tonsillar ectopia can not be used as the sole criteria for the diagnosis of the Chiari 1 malformation……it is likely that the position of the cerebellar tonsil although providing a general index of hind brain overcrowding is only one factor influencing the clinical features of Chiari 1 malformation."(12). Our clinical experience would confirm their observation that relying strictly on the degree of tonsillar ectopia may be too insensitive in the face of symptoms and neurological signs referable to the cervicomedullary junction. Furthermore, as the cerebellar tonsil is a parasagittal and not a mid-sagittal structure, it may be misleading to focus on the mid-sagittal MRI slice; the apparent tonsillar herniation will be greater in the parasagittal plane. As shown in figure 1(d and e), it is possible for the tonsils to appear above the foramen magnum on the mid sagittal MRI slice in the face of evidence for herniation in the parasagittal plane. However accurate measurement of the displacement relative to the rim of the foramen magnum is difficult on the parasagittal images. Our experience suggest that the visual impression of crowding is best appreciated on axial T-2 images angled directly through the plane of the foramen magnum. Conventional axial MRI images are often orthogonally oriented relative to the long axis of the patient while the plane of the foramen magnum is often oblique to the long axis of the patient. The contents of the foramen magnum will not actually be imaged unless properly oriented slices are obtained. The absence of the large cerebrospinal fluid space which normally surrounds the neural structures at this level of the neural axis will be most apparent on properly oriented images through the plane of the foramen magnum. In a series of 225 patients, Barkovich et al noted that as little as 1 mm of tonsillar ectopia resulted in narrowing or complete loss of the CSF space at the foramen magnum(3). Cerebrospinal fluid displacement is the earliest accommodation to craniocerebral disproportion. Quantitative cine-mode magnetic resonance imaging has been used to evaluate the velocity and direction of CSF flow in patients with Chiari type l malformation (tonsillar ectopia>5mm). Impaired caudal flow of CSF during systole and normalization following decompressive surgery have been demonstrated (1). Milhorat et al have documented similar findings in patients with less than 5 mm of tonsillar ectopia(12). However, in the absence of a prospective study, it cannot be determined if such an observation would be useful for selecting patients to undergo decompressive surgery. The pattern of CSF flow may be no more informative than the position of the cerebellar tonsil. We did not use this imaging technique.

In patients who present with signs and symptoms referable to the upper cervical spinal cord or caudal brainstem, our protocol calls for standard MR imaging of the cervical spine and brain supplemented with contiguous 3 mm thick T-2 weighted axial sections oriented through the plane of the foramen magnum. The concept of the disproportionately small posterior fossa as the principal cause of the tonsillar ectopia has been previously described(12,13,14,18,21). We suggest that the Chiari type 1 malformation should be viewed as a relative disproportion between the cross sectional area of the foramen magnum and the structures coursing through it rather than strictly as a measure of tonsillar ectopia. If the structures within the foramen magnum are crowded, then the patient may be symptomatic in the face of a minor degree of tonsillar ectopia. Surgical decompression may be appropriate in selected cases.

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Figures

Figure 1. Click on images for enlarged view.


a	b	c	d	e

(a) topogram showing the conventional orientation of axial MRI images. Note that the slices are not oriented in the plane of the foramen magnum.
(b) topogram showing the orientation of axial 3mm thick MRI slices in the plane of the foramen magnum in accordance with our protocol.
(c) T2 weighted axial MRI slice through the plane of the foramen magnum. Note that there is ample csf surrounding the brain stem and vertebral arteries with no evidence of tonsillar ectopia. This patient complained of headaches but did not have the physical findings often associated with compression of the neuraxis at this level.
(d) T2 weighted axial image through the plane of the foramen magnum in a patient whose symptoms were consistent with a Chiari type-1 malformation. The neurological examination was remarkable for diffuse hyper-reflexia and a left Babinski response. Note that the cerebellar tonsils are herniated into the lateral foramen magnum. CSF is evident in the dorsal midline(arrow).The ventral medulla oblongata(m) is deformed by the vertebral arteries which are applied to it. In accordance with our criteria, this foramen magnum would be considered crowded.
(e) The midline sagittal MRI image shows the cerebellar tonsils to be just above the foramen magnum.

Figure 2. Click on images for enlarged view.


a	b

(a) T1 weighted mid-sagittal MRI image of the brain showing a trivial degree of tonsillar ectopia measured to be 2.2mm below the inferior rim of the foramen magnum.
(b) The axial MRI slice through the foramen magnum shows much more extensive tonsillar herniation in the parasagittal plane with the appearance of crowding. It can be appreciated that a mid-sagittal section would show a generous subarachnoid space(arrow). The patient suffered from headache, dizziness and vertigo aggravated by extension of the neck. Lasting relief followed foramen magnum decompression.

Figure 3. Click on image for enlarged view.

T1 weighted mid-sagittal MRI image of the brain showing tonsillar ectopia which measured 2.9mm below the inferior rim of the foramen magnum. The patient suffered from incapacitating vertigo with extension of the neck. Lasting relief followed foramen magnum decompression.

Figure 4. Click on images for enlarged view.


a	b

(a) T2 weighted mid-sagittal MRI image showing the cerebellar tonsil to be located within the foramen magnum, i.e. 0 mm of ectopia.
(b) T1 weighted axial MRI image through the plane of the foramen magnum showing the tonsillar ectopia(t) and crowding of the contents of the foramen. The patient reported resolution of headache, nausea, vomiting, vertigo, and short term memory impairment following surgical decompression of the foramen magnum.